Direct electric heating (DEH) of long flowlines and large export pipelines provides many advantages compared to alternative methods. DEH has been applied actively in the past 10 years to prevent hydrate formation and is now breaking new ground that was not previously being considered feasible. By using qualified technology and existing design models, longer and larger pipelines can be heated intermittently or continuously.
Direct Electrical Heated Pipe in Pipe (DEHPIP) is a slightly different technological approach to the same problem that have quite similar demands for the electrical power supply system to drive them, hence most of the electrical energy supply system topologies can be used to power both DEH and DEHPIP systems independent of end-fed or midpoint fed topologies. Common for both systems is that the electric current flows axially through the pipe wall causing direct heating of the pipeline.
Wet-insulated: Open Loop System
                End-Fed Pipe        Center-Fed PipeDry-insulated: Closed Loop System        End-Fed Pipe-in-Pipe        Center-Fed Pipe-in-Pipe        
DEHPIP systems are sometimes described as Electrical Flowline Heating (EFH) systems since EFH systems traditionally have been associated with the Dry Insulated (Pipe-in-Pipe) flowline heating system technology, but the term can also be used as a general reference to any flowline heating using electricity.
Electric Heating of Pipelines is attractive for short and long step outs as DEH operating costs are considerably reduced compared to the use of chemicals. The technology is unique and commercially and technically attractive. It allows for the use of DEH for both infield flowlines, tie-backs and export pipelines with diameters around 6″ to 30″ and above. An increased number of DEH assemblies has been evaluated for oil and gas fields or project developments concepts around the world and the extension of this new technology will generally give higher flexibility in operation of the fields during planned or unplanned shut downs. Material aging and other failure mechanisms caused by high temperatures and water pressure are also of great importance. Accuracy in design and analysis as well as industry experience are important in solving project specific hydrate or wax issues in long DEH systems.
Using DEH can involve arranging a DEH cable along a steel pipeline. Current is guided through the DEH cable in one direction and returned through the pipeline steel in the return direction. Heat is generated in the pipeline steel, partly due to ohmic resistance in the steel and partly due to induced heat, as the current is an alternating current. As the contact between the DEH cable and the pipeline steel is not insulated from the surrounding sea water, a fraction of the current will also flow through the sea water and not in the pipeline.
Patent application publication EP2166637 (Siemens Aktiengesellschaft) describes a power supply arrangement for direct electrical heating (DEH) of a pipeline system. The power supply arrangement has a three phase transformer and a compensation unit including a capacitor means, and is adapted to feed electrical power to a single phase load.
WO2007011230 (Aker Kværner Engineering & Technology) describes a system for power supply to a flowline heating circuit. An electric distribution cable (3) is connected to the system, which extends to the subsea located pipeline (4) which is to be heated. In a subsea location there are arranged 3-to-2 phase transformers which connect electric power from a supply cable to sections of “piggyback” cables strapped onto the heated pipeline.
WO2006075913 describes a system for power supply to subsea installations, comprising electric power supply cables for DEH of a pipeline. The system is configurable to provide 3-phase power supply to an electric motor arranged subsea, when not heating the pipeline.